Photolithography is the transfer of a pattern to a photosensitive material by selective exposure to a radiation source such as, for example, light. A photosensitive material generally experiences a change in its material property when exposed to a radiation source. Typically, a photosensitive material includes a photoresist polymer or resist. After a resist is exposed to a radiation source of a specified wavelength and a developer solution, the chemical resistance of the resist changes and the resist will etch away either the exposed or unexposed regions, depending on the chemical properties of the resist and the developer solution. For example, if the exposed material is etched away by the developer and the unexposed region is resilient, the material is considered a positive resist. On the other hand, if the exposed material is resilient to the developer and the unexposed region is etched away, the material is considered a negative resist. Using the properties of the resist and the developer, patterns may be etched onto the surface of a wafer. Moreover, patterns may be used as a template for depositing materials after lithography. At the end of the process, the resist is typically etched away and any materials deposited on the resist is also etched away. Resists, however, cannot withstand high temperatures and may act as a source of contamination. Often times, etching near the edge of the wafer is simply not possible or not etched adequately within system parameters.
In most applications, the lithography process typically follows several standard steps to ensure that a wafer is etched accurately. For example, the lithography process typically includes preparing the surface of the wafer by baking the wafer to ensure that the resist will adhere properly. Some applications require that the wafer surface be prepared with an adhesion promoter. The wafer is spinned or sprayed uniformly with the resist and then soft baked to remove some of the solvent in the resist, making the resist more viscous. The wafer is typically aligned with a mask, selectively exposed to a radiation source and then baked again. Then, the wafer is exposed to a developer to selectively remove the resist. Finally, the wafer is typically hard baked to drive off more of the solvent in the resists and any resist residue is removed.
Prior art systems and methods, however, have failed to employ systems to enhance performance on the edge of the wafer. Even after selective imaging, prior systems often waste valuable wafer surface space (i.e., wafer edges) and are tedious and time consuming. Accordingly, the images were, at best, placed without taking into account most imaging practices, especially near the edge of the wafer. In addition, selective hard masking is currently accomplished by performing a photo process, an etch process, an ash or strip process. Often times, prior practices led to incomplete nodes and thus, the percentage of operational devices manufactured, or “yield”, is relatively low or simply unacceptable.
What is needed therefore is an improved, low cost method for hard masking wafers, including wafer edges.